Co-modulation of DBR laser and integrated optical amplifier

ABSTRACT

In an embodiment, a laser chip includes a laser, an optical amplifier, a first electrode, and a second electrode. The laser includes an active region. The optical amplifier is integrated in the laser chip in front of and in optical communication with the laser. The first electrode is electrically coupled to the active region. The second electrode is electrically coupled to the optical amplifier. The first electrode and the second electrode are configured to be electrically coupled to a common direct modulation source.

CROSS-REFERENCE TO RELATED APPLICATIONS

This patent application claims the benefit of and priority to U.S.Provisional Patent Application Ser. No. 61/648,499, filed May 17, 2012,which is incorporated herein by reference.

FIELD

The embodiments discussed herein relate to a laser chip including aco-modulated laser and integrated optical amplifier.

BACKGROUND

Lasers have become useful in a number of applications. For example,lasers may be used in optical communications to transmit digital dataacross a fiber-optic network. Directly modulated lasers (DMLs) may bemodulated by a modulation signal, such as an electronic digital signal,to produce an optical signal transmitted on a fiber-optic cable. Anoptically sensitive device, such as a photodiode, is used to convert theoptical signal to an electronic digital signal transmitted through thefiber-optic network. Such fiber-optic networks enable modern computingdevices to communicate at high speeds and over long distances.

In fiber-optic networks, DMLs are typically implemented as directlymodulated distributed feedback (DFB) lasers. Direct amplitude modulation(AM) of DFB lasers and other semiconductor lasers also results infrequency modulation (FM) of the DFB. At high-speeds, e.g., data ratesover 10 gigabits per second (G), The FM component of a high-speedoptical signal generated by a DFB laser closes the eye aftertransmission through dispersive fiber such that the reach of a 10 G DFBlaser is typically limited to about 5-10 kilometers (km).

The subject matter claimed herein is not limited to embodiments thatsolve any disadvantages or that operate only in environments such asthose described above. Rather, this background is only provided toillustrate one exemplary technology area where some embodimentsdescribed herein may be practiced.

BRIEF SUMMARY OF SOME EXAMPLE EMBODIMENTS

Embodiments described herein generally relate to a laser chip includinga co-modulated laser and integrated optical amplifier.

In an example embodiment, a laser chip includes a laser, an opticalamplifier, a first electrode, and a second electrode. The laser includesan active region. The optical amplifier is integrated in the laser chipin front of and in optical communication with the laser. The firstelectrode is electrically coupled to the active region. The secondelectrode is electrically coupled to the optical amplifier. The firstelectrode and the second electrode are configured to be electricallycoupled to a common direct modulation source.

Additional features and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by the practice of the invention. Thefeatures and advantages of the invention may be realized and obtained bymeans of the instruments and combinations particularly pointed out inthe appended claims. These and other features of the present inventionwill become more fully apparent from the following description andappended claims, or may be learned by the practice of the invention asset forth hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

To further clarify the above and other advantages and features of thepresent invention, a more particular description of the invention willbe rendered by reference to specific embodiments thereof which areillustrated in the appended drawings. Understanding that these drawingsdepict only several embodiments in accordance with the disclosure andare, therefore, not to be considered limiting of its scope, thedisclosure will be described with additional specificity and detailthrough use of the accompanying drawings, in which:

FIG. 1A illustrates an example embodiment of a distributed Braggreflector (DBR) laser implemented as a front DBR laser;

FIG. 1B illustrates an example embodiment of a DBR laser implemented asa rear DBR laser;

FIG. 1C illustrates an example embodiment of a DBR laser implemented asa front/rear DBR laser;

FIG. 1D is a cross-section of an example DBR laser through a gainsection of the DBR laser;

FIG. 1E illustrates an example frequency spectrum 140 of a directlymodulated DBR laser;

FIG. 2A illustrates a rate equations model for a laser, such as the DBRlasers of FIGS. 1A-1D;

FIG. 2B shows a model of a theoretical simulation to investigate carriertransport effect;

FIG. 3A illustrates eye diagrams and FM profiles associated with a firstexample directly modulated DBR laser;

FIG. 3B illustrates eye diagrams and FM profiles associated with asecond example directly modulated DBR laser;

FIG. 3C illustrates eye diagrams and FM profiles associated with a thirdexample directly modulated DBR laser;

FIG. 4A illustrates simulated AM and FM profiles for the DBR laser ofFIG. 3A for a 10 G binary bit sequence;

FIG. 4B illustrates eye diagrams and a measured 21 km AM profile for theDBR laser of FIG. 3A;

FIG. 5A illustrates simulated AM and FM profiles for the DBR laser ofFIG. 3C for the same 10 G binary bit sequence as FIG. 4A;

FIG. 5B illustrates a measured back-to-back (BB) AM profile, a measured21 km AM profile, and eye diagrams for the DBR laser of FIG. 3C;

FIG. 5C illustrates an example electrical high-pass filter that may beimplemented to equalize a low frequency issue, or slow component, in theDBR laser of FIG. 3C;

FIGS. 6A-6G illustrate simulations with various DBR parameters;

FIG. 7 illustrates eye diagrams for a directly modulated DFB laser andeye diagrams for a directly modulated DBR laser;

FIG. 8 illustrates receive power for various extinction ratios (ERs)changed by adjusting modulation amplitude for fixed DC bias for adirectly modulated DBR laser;

FIG. 9 illustrates relative damping for a DFB laser and a DBR laser;

FIG. 10 is a graph depicting delay time, mirror slope, and detuneloading alpha, all as a function of wavelength, for an exampleembodiment of a DBR laser;

FIG. 11 is a graph depicting measured FM efficiency as a function ofphase condition of an example embodiment of a DBR laser;

FIG. 12A illustrates an example embodiment of a laser chip including aDBR laser and an integrated optical amplifier;

FIG. 12B illustrates an example embodiment of the laser chip of FIG. 12Ain which the optical amplifier includes a semiconductor opticalamplifier (SOA);

FIG. 12C illustrates an example embodiment of the laser chip of FIG. 12Ain which the optical amplifier includes a multimode interference (MMI)SOA;

FIG. 12D illustrates an example embodiment of a first N-arm MZ modulatorthat may be implemented in the laser chip of FIG. 12C;

FIG. 12E illustrates an example embodiment of a second N-arm MZmodulator that may be implemented in the laser chip of FIG. 12C;

FIG. 13 is a graph including an AM profile of an optical signaltransmitted through a SOA with high bias;

FIG. 14 illustrates an AM profile and an FM profile of an optical signalgenerated by a directly modulated laser after passing through a SOAunder the same conditions as described with respect to FIG. 13;

FIG. 15 illustrates 3 cases for operation of a DBR laser and a SOA; and

FIG. 16 is a graph indicating operating conditions that can be used toavoid ER degradation after a SOA.

DETAILED DESCRIPTION OF SOME EXAMPLE EMBODIMENTS

Embodiments described herein generally relate to high-speed directlymodulated lasers (DMLs) with reach of 20 km or more and/or which may besuitable for passive optical network (PON) applications. Exampleembodiments may include distributed feedback (DFB) and/or distributedBragg reflector (DBR) lasers with relatively thick SCH to damp ringingin a frequency response of the lasers. Alternately or additionally,example embodiments may include laser chips including a laser and anintegrated optical amplifier that are co-modulated with the samemodulation signal.

Reference will now be made to the drawings to describe various aspectsof example embodiments of the invention. It is to be understood that thedrawings are diagrammatic and schematic representations of such exampleembodiments, and are not limiting of the present invention, nor are theynecessarily drawn to scale.

FIG. 1A illustrates an example embodiment of a DBR laser 100, arrangedin accordance with at least some embodiments described herein. Ingeneral, the DBR laser 100 includes a gain section 101 and a passivesection 102. The gain section 101 includes an active region 103 such asa multiple quantum well (MQW) region. The passive section 102 is coupledto the gain section 101 and may include a DBR 104 in opticalcommunication with the active region 103. The DBR laser 100 additionallyincludes a gain electrode 105 coupled to the gain section 101.

The DBR laser 100 may additionally include a high reflectivity (HR)coating 106 applied to a rear of the DBR laser 100 and/or anantireflection (AR) coating 107 applied to a front of the DBR laser 100.

As generally depicted at 108, the DBR laser 100 may be tuned to a longwavelength side of a Bragg peak associated with the DBR 104 operating intransmission. Direct modulation of the DBR laser 100 may generate anoptical signal with both frequency modulation (FM) and amplitudemodulation (AM). Accordingly, tuning the DBR laser 100 to the longwavelength side of the Bragg Peak may attenuate longer-wavelength zerobits less than shorter-wavelength one bits, thereby reducing theextinction ratio (ER) of the optical signal emerging from the front ofthe DBR laser 100 compared to the optical signal emitted by the gainsection 101 as it enters the passive section 102.

The DBR laser 100 illustrated in FIG. 1A is an example of a front DBRlaser, e.g., a DBR laser in which the passive section 102 including theDBR filter 104 is positioned between the gain section 101 and the frontof the DBR laser 100. Embodiments described herein may additionallyinclude rear DBR lasers and/or front/rear DBR lasers as illustrated inFIGS. 1B-1C.

FIG. 1B illustrates an example embodiment of a DBR laser 110 implementedas a rear DBR laser, arranged in accordance with at least someembodiments described herein. The DBR laser 110 includes many componentsand/or structures that are analogous to corresponding components and/orstructures of the DBR laser 100 of FIG. 1A. Briefly, for example, theDBR laser 110 includes a gain section 111 and a passive section 112. Thegain section 111 includes an active region 113 such as a MQW region. Thepassive section 112 is coupled to the gain section 111 and may include aDBR 114 in optical communication with the active region 113.

The DBR laser 110 additionally includes a gain electrode 115 coupled tothe gain section 111, an HR coating 116 applied to a rear of the DBRlaser 110, and an AR coating applied to a front of the DBR laser 110.Because the DBR laser 110 is a rear DBR laser, the passive section 112including the DBR 114 is positioned at the rear of the DBR laser 110with the gain section 111 being positioned between the passive section112 and the front of the DBR laser 110.

As generally depicted at 118, the DBR laser 110 may be tuned to a longwavelength side of a Bragg peak associated with the DBR 114 operating inreflection. In this example, tuning the DBR laser 110 to the longwavelength side of the Bragg Peak may attenuate longer-wavelength zerobits more than shorter-wavelength one bits, thereby enhancing theextinction ratio (ER) of the optical signal emerging from the front ofthe DBR laser 110 compared to the optical signal emitted by the gainsection 111 as it enters the passive section 112.

In both front and rear DBR lasers, there is a natural tendency to laseron the long wavelength side of the associated Bragg peak due to crossgain compression (e.g., gain material non-linear effect). Additionally,short-cavity (SC) front or rear DBR lasers can be implemented in whichlasing can be forced to happen on the short wavelength side of the Braggpeak. However, in both front and rear DBR lasers, the laser is faster onthe long wavelength side of the Bragg peak. As will be explained in moredetail below, the lasing position of the DBR with respect to the Braggpeak, or the tuning of the DBR laser, also referred to as thedetune-loading effect, can be leveraged to improve the performance ofthe DBR laser.

FIG. 1C illustrates an example embodiment of a DBR laser 120 implementedas a front/rear DBR laser, arranged in accordance with at least someembodiments described herein. The DBR laser 120 includes many componentsand/or structures that are analogous to corresponding components and/orstructures of the DBR lasers 100, 110 of FIGS. 1A and 1B. Briefly, forexample, the DBR laser 120 includes a gain section 121, a front passivesection 122A, and a rear passive section 122B. The gain section 121includes an active region 123 such as a MQW region. The front passivesection 122A is coupled to the gain section 121 and may include a frontDBR 124A in optical communication with the active region 123.Analogously, the rear passive section 122B is coupled to the gainsection 121 and may include a rear DBR 124B in optical communicationwith the active region 123. Because the DBR laser 120 is a front/rearDBR laser, gain section 121 may be positioned between the front passivesection 122A including the DBR 124A and the rear passive section 122Bincluding the DBR 124B.

The DBR laser 120 additionally includes a gain electrode 125 coupled tothe gain section 121, an HR coating 126 applied to a rear of the DBRlaser 120, and an AR coating applied to a front of the DBR laser 120. Asgenerally depicted at 128A, the DBR laser 120 may be tuned to a shortwavelength side of a Bragg peak associated with the front DBR 124A. Asgenerally depicted at 128B, the DBR laser 120 may be tuned to a longwavelength side of a Bragg peak associated with the rear DBR 124B.

In some embodiments, the rear DBR filter 124B may have a relatively highreflectance of 90% or more, while the front DBR filter 124A may have arelatively low reflectance in a range from 5% to 30%. A lasing mode ofthe DBR laser 120 may be controlled by adjusting relative phases of thefront and rear DBR filters 124A-124B. With an appropriate relative phaseshift, yield for single mode lasing may be increased to nearly 100%. Inthese and other embodiments, a side-mode suppression ratio (SMSR) may bemainly determined by the rear DBR filter 124B with the higherreflectivity which therefore provides larger difference in thresholdgain for main mode and second side mode. The Bragg peak of the front DBRfilter 124A may be shifted to the shorter wavelength side as alreadydescribed above so that the ER after passage through the front DBRfilter 124A increases the ER, and without degrading SMSR since SMSR ismainly determined by the rear DBR filter 124B. Optionally, the activeregion 123 may include a grating at the center in order to improve SMSRand single mode yield, in which case the DBR laser 120 may instead beimplemented as a distributed Bragg (DR) laser. The detune-loading effectmay be reduced since dynamic change in threshold gain will be reduced bythe presence of the grating in the active region 123.

As already described above, each of the DBR lasers 100, 110, 120 ofFIGS. 1A-1C includes a gain electrode 105, 115, 125 coupled to acorresponding gain section 101, 111, 121. The gain electrode 105, 115,125 is configured to be coupled to a direct modulation source (notshown), such as a laser driver. The direct modulation source may providea modulation signal having a data rate of about 10 gigabits per second(“G”) or higher. As used herein, the term “about” as applied to a valuemay be interpreted as the value plus or minus 10% of the value. Thus,the gain section 101, 111, 121 may be directly modulated by a modulationsignal having a data rate of about 10 G or higher. More generally, allof the lasers described herein may be modulated at a data rate of about10 G or higher.

In these and other embodiments, the modulation signal applied to thegain section 101, 111, 121 may have a modulation swing of at least 40milli amps peak-to-peak (mApp) and/or the gain section 101, 111, 121 mayhave a length of 300 micrometers (um) or less. In an exampleimplementation, the modulation swing may be about 60 mApp and the gainsection 101, 111, 121 may have a length of about 200 um. As anotherexample, the modulation swing may be 90 mApp or more. Alternately oradditionally, a modulation density—defined as the modulation swingdivided by the length of the gain section 101, 111, 121—may be 0.2mApp/um or more.

FIG. 1D is a cross-section of an example DBR laser 130 through a gainsection of the DBR laser, arranged in accordance with at least someembodiments described herein. Each of the DBR lasers 100, 110, 120 ofFIGS. 1A-1C may have an identical or similar configuration as the DBRlaser 130 of FIG. 1D.

In general, the DBR laser 130 includes an active region 131. The activeregion 131 may be formed from a quaternary material, such asindium-gallium-arsenide-phosphide (InGaAsP) orindium-gallium-alluminum-arsenide (InGaAlAs). Alternately oradditionally, the active region 131 may include an MQW region, such asan InGaAsP or InGaAlAs MQW strained layer, having 5-12 quantum wells(QWs) and corresponding barriers. For instance, the MQW region mayinclude 8 QWs and 7 barriers.

The DBR laser 130 further includes an upper separate confinementheterostructure (SCH) 132 and a lower SCH 133. In the illustratedembodiment, the upper SCH 132 includes a lower layer 132A of a firstmaterial, and an upper layer 132B of a second material. The lower SCH132 includes an upper layer 133A of the first material, and a lowerlayer 133B of the second material. Each of the first and secondmaterials may include a quaternary material such as InGaAsP or InGaAlAs,or the like, or other suitable material(s). Although each SCH 132 and133 is illustrated as being made up of two layers 132A and 132B or 133Aand 133B, more generally, each SCH 132 and 133 may include one or morelayers.

In an example embodiment, the lower layer 132A of the upper SCH 132 maybe undoped, while the upper layer 132B of the upper SCH 132 is p-dopedwith a doping density in a range from about 2×10¹⁷ cm⁻³ to about 5×10¹⁷cm⁻³. Analogously, the upper layer 133A of the lower SCH 133 may beundoped, while the lower layer 133B may be n-doped with a doping densityof about 2×10¹⁷ cm⁻³ to about 5×10¹⁷ cm⁻³.

Each of the upper and lower SCHs 132, 133 may have a thickness of atleast 60 nanometers (nm). Alternately or additionally, each of the upperand lower SCHs 132, 133 may have a thickness less than 120 nm.

The DBR laser 130 further includes a substrate 134 on which the layers131-133 are formed. The substrate 134 may be made of the secondmaterial. Alternately or additionally, the substrate 134 may be n-dopedwith a doping density in a range from about 1×10¹⁸ cm⁻³ to about 3×10¹⁸cm⁻³.

The DBR laser 130 may additionally include a cladding layer 135, anarrow bandgap layer 138, an upper or gain electrode 136, and a lowerelectrode 137. The cladding layer 135 may include p-dopedindium-phosphide (InP). The narrow bandgap layer 138 may include, forinstance, a thin layer of highly p-doped indium-gallium-arsenide(InGaAs) to improve ohmic contact.

FIG. 1E illustrates an example frequency spectrum 140 of a directlymodulated DBR laser, arranged in accordance with at least someembodiments described herein. For instance, any of the DBR lasers 100,110, 120, 130 of FIGS. 1A-1D may have a frequency spectrum similar tothe frequency spectrum 140 of FIG. 1E when directly modulated. Indirectly modulated lasers (DMLs) that are amplitude modulated, frequencymodulation also occurs in a manner that generally follows the amplitudemodulation profile, such that 1 bits are typically blue-shifted relativeto 0 bits (or vice versa when modulated by data-bar), as illustrated inFIG. 1E.

Such frequency excursion between the 1 bits and 0 bits is often referredto as chirp. The frequency spectrum 140 of FIG. 4 has about 30 gigahertz(GHz) of chirp. More generally, the chirp may be in a range from about0.1 GHz/mApp to about 0.5 GHz/mApp. The frequency spectrum 140 of FIG. 4including 30 GHz of chirp was generated for a DBR laser directlymodulated by a 60 mApp modulation signal.

FIG. 2A illustrates a rate equations model 200 for a laser, arranged inaccordance with at least some embodiments described herein. The rateequations model 200 may be used for the directly modulated DBR lasersdescribed herein, for instance. The rate equations model 200 includes acarriers rate equation 202, a photons rate equation 204, and a phaserate equation 206. In equations 202, 204, 206:

-   -   N is carriers including electrons and holes in the active        region;

$\frac{\partial N}{\partial t}$is the carrier density;

-   -   I is total modulation current, including the sum of direct        current (DC) bias and modulation swing;    -   g is gain and is a function of N;    -   ε is gain compression factor and is a constant material        parameter;    -   S is photons in the active region;

$\frac{\partial S}{\partial t}$is the change in S over time;

-   -   τ_(v) is the photon lifetime;    -   v is frequency of the generated optical signal;

$\frac{\partial\varphi}{\partial t}$is the change in phase of photons over time, which is equal to frequencyv;

-   -   Γ_(well) is the optical confinement factor for the MQW region;    -   α is linewidth enhancement factor and is a constant material        parameter;    -   Γ_(SCH) is the optical confinement factor for the SCH;    -   dn/dN_(SCH) is the change in diffractive index due to change in        carrier density in the SCH, also referred to as the differential        diffractive index change due to carrier change in the SCH; and    -   N_(SCH) is carriers including electrons and holes in the SCH.

In equation 206, gain g is indicated to be a function of N. Similarly,gain g in equations 202 and 204 should be a function of N.

In conventional rate equations models, the photon lifetime τ_(p) isconsidered to be constant for simplicity. According to embodimentsdescribed herein, however, the photon lifetime τ_(p) is considered tochange dynamically. In particular, the photon lifetime τ_(p) is ameasure of photon loss, including carrier-created loss such as plasmaeffect of loss or intervalance band absorption (IVBA), or some otherabsorption mechanism related to the injection carrier (e.g., electrons).The carrier-created loss dynamically changes when the laser is directlymodulated because the injection carrier is directly modulated.Additionally, in DBR lasers, the reflectance (e.g., mirror loss) of theDBR filter is frequency-dependent. Thus, the reflectance or mirror lossof the DBR filter will vary as a function of the frequency of thegenerated photons, which is generally higher for 1 bits than for 0 bitsfor a DML, as described with respect to FIG. 1E.

The rate equations model 200 additionally takes into account carriertransport effect. In general, the carrier transport effect refers to theeffect on optical output as a result of the delays involved intransporting carriers through various layers of a DML. FIG. 2B shows amodel 210 of a theoretical simulation to investigate the carriertransport effect, arranged in accordance with at least some embodimentsdescribed herein. Although a brief description of the model 210 will beprovided herein, more detailed modeling can be found in Matsui et al.,Novel Design Scheme for High-Speed MQW Lasers with Enhanced DifferentialGain and Reduced Carrier Transport Effect, IEEE Journal of QuantumElectronics, Vol. 34, No. 12, December 1998, which publication is hereinincorporated by reference.

The model 210 of FIG. 2B includes a cladding layer, an SCH, two QWs, anda barrier that may all be included in a DML, such as a directlymodulated DBR laser as generally described above. When directlymodulated, carrier is injected through the cladding layer, goes throughthe SCH layer, is captured by the QW, may escape the QW and/or maytunnel through the barrier to another QW before a transition occurs.Each one of the foregoing steps takes some finite time and creates adelay which may damp ringing or relaxation oscillation. Moreover, inFIG. 2B:

-   -   τ_(dif) is SCH diffusion lifetime;    -   τ_(drift) is SCH drift lifetime;    -   τ_(cap) is QW capture lifetime;    -   τ_(esc) is QW escape lifetime; and    -   τ_(tun) is barrier tunneling lifetime;        In some embodiments, τ_(dif) and τ_(drift) are on the order of a        few ps so they do not significantly reduce the speed of the        laser.

FIG. 2B further illustrates the relationship between 2D carriers in thewells and 3D carriers in extended state in barriers in equation 212. Thedenominator 3D/2D in equation 212 may be determined by the Boltzmanndistribution.

In some embodiments, the carrier transport effect is accounted for thein the rate equations model 200 of FIG. 2A by, in general, determiningcarrier density in the QWs according to the model 210 of FIG. 2B tocharacterize gain g, and then solving the equations 202, 204, 206 ofFIG. 2A. In more detail, in FIG. 2A, gain g is a function of N. In FIG.2B, 2D represents the carriers confined in two dimensions in each of theQWs. Carriers N are injected from the electrode through the claddinglayer, transport through the SCH with time constants τ_(dif) andτ_(drift) and reach the barrier as the 3D state before being captured inthe QWs. After reaching the QWs, the carriers N may contribute to gain,although some carriers N may escape by tunneling or thermionic emission.Accordingly, the model of FIG. 2B can be solved to find carrier densityin the well, to characterize gain g, and to thereby solve equations 202,204, 206 of FIG. 2A.

FIG. 3A illustrates eye diagrams 302, 304 and FM profiles 306, 308associated with an example directly modulated DBR laser V3, arranged inaccordance with at least some embodiments described herein. The DBRlaser V3 is a 10 G directly modulated front DBR laser such as the DBRlaser 100 of FIG. 1A. FIG. 3A further illustrates a table 310 includingvarious parameters of the DBR laser V3. In particular, according to thetable 310, the DBR laser V3 has an SCH thickness of 100 nm, a barrier of1.2 Q, a MQW PL of 1550 nm, and a K factor of 0.32 nanoseconds (ns).

The eye diagram 302 is a 20 kilometer (km) simulated optical eyediagram, while the eye diagram 304 is a 20 km measured optical eyediagram. As shown, the eye diagram 304 is substantially open even aftertransmission through 20 km of dispersive fiber.

The FM profile 306 is a simulated FM profile for the DBR laser V3 basedon the rate equations model 200 of FIG. 2A, while the FM profile 308 isa measured FM profile for the DBR laser V3. Both FM profiles 306, 308include a 0 1 0 bit sequence. As illustrated, both FM profiles 306, 308have about 35 GHz of adiabatic chirp and 10 GHz of transient chirp.Thus, a ratio of transient chirp to adiabatic chirp of 2:7 (or about0.286) produces a 20 km eye that is substantially open, as shown in theeye diagram 304. More generally, 10 G and higher directly modulated DBRlasers as described herein may have a ratio of transient chirp toadiabatic chirp in a range from 1:3 to 1:4 (or a range from about 0.33to 0.25), which may be suitable to produce a 20 km eye that issubstantially open.

FIG. 3B illustrates eye diagrams 312, 314 and FM profiles 316, 318associated with an example directly modulated DBR laser V4, arranged inaccordance with at least some embodiments described herein. The DBRlaser V4 is a 10 G directly modulated front DBR laser such as the DBRlaser 100 of FIG. 1A. FIG. 3B further illustrates a table 320 includingvarious parameters of the DBR laser V4. In particular, according to thetable 320, the DBR laser V4 has an SCH thickness of 60 nm, a barrier of1.2 Q, a MQW PL of 1580 nm, and a K factor of 0.23 ns.

The FM profile 316 is a simulated FM profile for the DBR laser V4 basedon the rate equations model 200 of FIG. 2A, while the FM profile 318 isa measured FM profile for the DBR laser V4. Both FM profiles 316, 318include a 0 1 0 bit sequence. As illustrated, both FM profiles 316, 318have about 35 GHz of adiabatic chirp and 17 GHz of transient chirp. Inthis case, the ratio of transient chirp to adiabatic chirp is too high,e.g., about 1:2 and the K factor is too low, producing a 20 km eye thatis substantially closed, as shown in the simulated and measured eyediagrams 312, 314.

FIG. 3C illustrates eye diagrams 322, 324 and FM profiles 326, 328associated with an example directly modulated DBR laser V5, arranged inaccordance with at least some embodiments described herein. The DBRlaser V5 is a 10 G directly modulated front DBR laser such as the DBRlaser 100 of FIG. 1A. FIG. 3C further illustrates a table 330 includingvarious parameters of the DBR laser V5. In particular, according to thetable 330, the DBR laser V5 has an SCH thickness of 125 nm, a barrier of1.2 Q, a MQW PL of 1610 nm, and a K factor of 0.34 ns.

The FM profile 326 is a simulated FM profile for the DBR laser V5 basedon the rate equations model 200 of FIG. 2A, while the FM profile 328 isa measured FM profile for the DBR laser V5. Both FM profiles 326, 328include a 0 1 0 bit sequence. As illustrated, both FM profiles 326, 328have about 35 GHz of adiabatic chirp and effectively no transient chirp.In this case, the ratio of transient chirp to adiabatic chirp iseffectively non-existent and the K factor is too high, producing a 20 kmeye that is substantially closed, as shown in the simulated and measuredeye diagrams 322, 324. The 20 km eye is closed due to a low-pass filtereffect caused by the thicker SCH compared to that of the DBR laser V3.As will be described in greater detail below, however, a high-passelectrical filter may be used to filter the modulation signal applied tothe DBR laser V5 to effectively cancel out the low-pass filter effect.

FIG. 4A illustrates simulated AM and FM profiles 402, 404 for the DBRlaser V3 of FIG. 3A for a 10 G binary bit sequence. As illustrated inFIG. 4A, the FM profile 404 generally follows the AM profile 402. FIG.4A additionally includes arrows 406, 408 identifying, respectively,adiabatic chirp and transient chirp.

The FM profile 404 generally remains flat, not counting the transientchirp, such that all the 1 bits have substantially the same frequencyand all the 0 bits have substantially the same frequency that is lowerthan the frequency of the 1 bits, even for 1 bits in 0-rich regionsand/or 0 bits in 1-rich regions of the binary bit sequence, therebyreducing dispersion as compared to FM profiles with FM droop. By way ofillustration, consider 0-rich region 410 preceded by a 1 bit andterminated by a 1 bit. The relative timing between the preceding 1 bitand the terminating 1-bit is depicted in FIG. 4A at 412 for acorresponding measured back-to-back (BB) AM profile 414.

Because the FM profile 404 generally remains flat, there is no timingskew between the preceding and terminating 1 bits after transmissionthrough 21 km of fiber, as illustrated by a corresponding measured 21 kmAM profile 416. In more detail, the preceding 1 bit and the terminating1 bit have the same frequency and thus travel through 21 km of fiberwith the same speed, thereby maintaining the same relative timing 412.

FIG. 4B illustrates eye diagrams 418, 420, 422 and the measured 21 km AMprofile 416 in more detail for the DBR laser V3 of FIG. 3A. The eyediagram 418 is a measured BB eye diagram, the eye diagram 420 is asimulated 20 km eye diagram, and the eye diagram 422 is a measured 21 kmeye diagram. It can be seen in FIG. 4B that both the BB eye and the 21km eye for the DBR laser V3 are substantially open.

FIG. 5A illustrates simulated AM and FM profiles 502, 504 for the DBRlaser V5 of FIG. 3C for the same 10 G binary bit sequence as FIG. 4A.FIG. 5A additionally identifies various features in the AM and/or FMprofiles 502, 504. The features include a notch in the FM profile 504before 1-to-0 transitions, FM droop in 0 rich areas withoutcorresponding AM droop, a dip after peaking in the FM profile 504 at0-to-1 transitions, FM undershoot at 1-to-0 transitions withoutcorresponding AM undershoot, weak damping in both the AM and FM profiles502, 504 in 1-rich regions, and single 0 bits that appear very fast.

FM droop may generally refer to a slow change in the FM profile 504,e.g., on the order of 1 ns, and is also referred to as the slowcomponent mentioned along with the weak damping feature identified inFIG. 5A. This slow component is not predicted by standard laser rateequations models in which the photon lifetime τ_(p) is consideredconstant and the carrier transport effect is not accounted for. Byaccounting for a dynamic photon lifetime and the carrier transporteffect in the rate equations model 200 of FIG. 2A as already describedabove, however, this slow component is predicted for the DBR laser V5 asa result of the SCH thickness being too great.

One issue with the slow component or FM droop is that it may skewtiming. In more detail, consider 0-rich region 506 preceded by a 1 bitand terminated by a 1 bit. Because the preceding 1 bit for the 0-richregion 506 terminates a 1-rich region 508, the slow component causes thefrequency of the preceding 1 bit for the 0-rich region 506 to berelatively high as the frequency of the 1 bits in the 1-rich region 508is generally increasing. Within the 0-rich region 506, however, the slowcomponent causes the frequency of the 0 bits in the 0-rich region todecrease significantly, or droop, and the resulting frequency of theterminating 1 bit for the 0-rich region 506 is therefore significantlylower than the frequency of the preceding 1 bit for the 0-rich region506. The frequency difference between the preceding and terminating 1bits for the 0-rich region 506 causes timing skew after transmissionthrough 21 km of dispersive fiber, as described in more detail withrespect to FIG. 5B.

FIG. 5B illustrates a measured BB AM profile 510, a measured 21 km AMprofile 512, eye diagrams 514, 516, 518, and more detailed versions510A, 512A of the measured BB and 21 km AM profiles 510, 512 for the DBRlaser V5 of FIG. 3C. With combined reference to FIGS. 5A-5B, therelative timing between the preceding 1 bit and the terminating 1 bitfor the 0-rich region 506 is depicted in FIG. 5B at 520 for the measuredBB AM profile 510. Because the terminating 1 bit has a significantlylower frequency than the preceding 1 bit for the 0-rich region 506, theterminating 1 bit will travel slower through 21 km of dispersive fiberthan the preceding 1 bit. In the present example, the terminating 1 bittraveled 40 picoseconds (ps) slower through the same 21 km length offiber as the preceding 1 bit. The longer traveling time is referred toas timing skew. More generally, however, a relative timing 522 betweenthe preceding 1 bit and the terminating 1 bit in the measured 21 km AMprofile 512 is greater than the relative timing 520 between thepreceding 1 bit and the terminating 1 bit in the measured BB AM profile510 due to the lower frequency of the terminating 1 bit compared to thepreceding 1 bit.

The eye diagram 514 is a measured BB eye diagram, the eye diagram 516 isa simulated 20 km eye diagram, and the eye diagram 518 is a measured 21km eye diagram. The timing skew as already described above closes theeye diagram 518 in an area generally denoted at 520 in the eye diagram518.

FIG. 5C illustrates an example electrical high-pass filter (HPF) 522that may be implemented to equalize the low frequency issue, or slowcomponent, in the DBR laser V5 of FIG. 3C caused by the thickness of theSCH, arranged in accordance with at least some embodiments describedherein. For example, the HPF 522 may be provided in an opticaltransmitter including a direct modulation source and the DBR laser V5 ora similar DBR laser. The HPF 522 may be coupled between the directmodulation source and the DBR laser.

In the illustrated embodiment, the HPF 522 is an RC circuit and includesa capacitor 524, a first resistor 526, and a second resistor 528. Thecapacitor 524 is coupled in parallel with the first resistor 526. Theparallel-coupled capacitor 524 and first resistor 526 are coupled inseries with the second resistor 528. The HPF 522 further includes aninput node 530 configured to be coupled to the direct modulation sourceand an output node 532 configured to be coupled to the DBR laser. In anexample embodiment, the capacitor 524 has a capacitance of about50-picofarads (pF), the first resistor 526 has a resistance of about 15Ohms (Ω), and the second resistor 528 has a resistance of about 45Ω.

A HPF with a similar time constant as the HPF 522 can be designed usinga parallel shunt inductor with an additional resistor in series with theinductor before being connected to ground.

FIG. 5C additionally illustrates various current responses 534 of theHPF 522 over time for three different resistance values for the firstresistor 526. With combined reference to FIGS. 5A and 5C, and as alreadyexplained above, in 0-rich regions such as the 0-rich region 506, the FMprofile 504 or the chirp gradually droops. As best seen with respect tothe current responses 534, the HPF 522 essentially does the opposite byinjecting current gradually over 0-rich regions, which may ultimatelysubstantially cancel out or equalize the FM droop. Accordingly, the HPF522 may have a time constant on the order of about 1 ns, or moregenerally a time constant about equal to the time constant of the slowcomponent.

FIG. 5C additionally illustrates eye diagrams 536 and 538. The eyediagram 536 is a measured BB eye diagram for the optical transmitterincluding the DBR laser V5 and the HPF 522. The eye diagram 538 is ameasured 21 km eye diagram for the optical transmitter including the DBRlaser V5 and the HPF 522. As can be seen by comparing the eye diagram538 of FIG. 5C to the eye diagram 518 of FIG. 5B, the HPF 522 equalizesthe low frequency issue or slow component and opens up the measured 21km eye diagram.

FIGS. 6A-6G illustrate simulations 600A-600G (collectively “simulations600”) with various DBR parameters, arranged in accordance with at leastsome embodiments described herein. Some of the simulations adjust theparameters consistent with the rate equations model 200 and/or the model210 of FIGS. 2A-2B.

Each of FIGS. 6A-6G includes two graphs, including a first graph withvarious time series simulations of a 1 0 1 bit sequence and DBR laserbias and modulation values, and a second graph with various distributionsimulations within certain layers of the DBR laser. The time seriessimulations include photon density (hereinafter “Photon”), chirpattributable to the MQW region (hereinafter “ChirpMQW”), chirpattributable to the plasma effect (hereinafter “ChirpPlasma”), and totalchirp (hereinafter “TotalChirp”) as the sum of ChirpMQW and ChirpPlasma.In some embodiments, ChirpMQW may be similar to gain, and ChirpPlasmamay relate to overflow carriers. The distribution simulations includeelectron density in the wells (hereinafter “Electron well”), holedensity in the wells (hereinafter “Hole well”), electron density in thebarriers (hereinafter “Electron barrier”), hole density in the barriers(hereinafter “Hole barrier”), and electric field (hereinafter “Electricfield”).

The parameters for the simulation 600A of FIG. 6A are provided in TableA below.

TABLE A Parameter: Value: electron capture time  0.3 ps hole capturetime  0.3 ps hole overflow → hole 0.1% escape time 300 ps electronmobility  2500 cm{circumflex over ( )}2/V/s hole mobility  2500cm{circumflex over ( )}2/V/s electron tunneling time 0.134 ps holetunneling time 0.134 ps

The simulation 600A of FIG. 6A is a relatively simple case where photonlifetime is considered to be constant. As can be seen from the timeseries simulations, damping is relatively small and there is substantialringing. As can be seen from the distribution simulations, there is arelatively uniform hole distribution.

The parameters for the simulation 600B of FIG. 6B are provided in TableB below.

TABLE B Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 0.1% escape time 300 ps electronmobility  2500 cm{circumflex over ( )}2/V/s hole mobility   180cm{circumflex over ( )}2/V/s electron tunneling time 0.134 ps holetunneling time  4.0 ps

Compared to the simulation 600A of FIG. 6A, the carrier transport effectis considered in the simulation 600B of FIG. 6B. For instance, theelectron capture time is increased from 0.3 ps to 1.0 ps, the holemobility is decreased from 2500 cm²/V/s to 180 cm²/V/s, and the holetunneling time is increased from 0.134 ps to 4.0 ps. The foregoingmodified parameters correspond to an SCH thickness of 100 nm. Thecarrier transport effect damps the ringing in the time seriessimulations of FIG. 6B.

The middle of the distribution simulations where the Electron welldensity and the Hole well density is greatest corresponds to theposition of a QW. As illustrated, holes are generally coming from theright and electrons are coming from the left and they combine in the QW.Because the holes are transported relatively slowly through the p-sideSCH to the right of the QW, the holes accumulate on the right of the QW.In the simulation 600A of FIG. 6A, carrier is injected directly into theQW so the carrier is very fast, resulting in undamped strong ringing. Inthe simulation 600B of FIG. 6B, however, the carrier transport effectresulting from injecting carrier through the SCH delays the carriers,and thereby damps the oscillator and reduces ringing.

Thus, in the simulation 600B of FIG. 6B, overflow in the p-side SCHincreases for both electrons and holes. The electrons are attracted bythe holes and overflow into the SCH for high bias condition. For the1-to-0 transition, electrons discharge from the SCH and flow back to theQW. There is very efficient damping of undershooting for the 1-to-0transition for AM, e.g., as can be seen from the damping inundershooting for the Photon time series. There is still undershootingfor the 1-to-0 transition for FM, e.g., as can be seen from theundershooting for the TotalChirp time series. The damping is not veryefficient for the 0-to-1 transition overshooting because speed ofcharging the SCH is slower than the relaxation oscillation frequencyf_(r).

In this and other embodiments, the relaxation oscillation frequencyf_(r) may be at least 12 GHz, or at least 16 GHz, or even higher.Alternately or additionally, in this and other embodiments, dampingcaused by carrier transport effect tin eh gain section may be at least12 GHz.

Conventional wisdom regarding high-speed (e.g., about 10 G or higher)DMLs is to limit SCH thickness to less than 50 nm, and more typically touse an SCH thickness of about 35 nm because the thicker SCH can negativeaffect the high-speed performance of the laser. In the embodimentsdescribed herein, however, SCH thickness of 60 nm to about 125 nm may beused to damp transient chirp and improve the eye after transmissionthrough dispersive fiber for both DBR lasers and/or DFB lasers. Thus,while the example embodiments generally described and/or depicted hereinrelate to DBR lasers, the principles disclosed herein may also apply toDFB lasers. For example, relatively thick SCH may be used in a DFB laserto damp transient chirp and improve the eye after transmission throughdispersive fiber.

The parameters for the simulation 600C of FIG. 6C are provided in TableC below.

TABLE C Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 10% escape time 3 ps electron mobility 2500 cm{circumflex over ( )}2/V/s hole mobility   30 cm{circumflex over( )}2/V/s electron tunneling time 0.134 ps hole tunneling time  4.0 ps

Compared to the simulation 600B of FIG. 6B, the carrier transport or SCHeffect is even stronger in the simulation 600C of FIG. 6C. For instance,the hole overflow is increased from 0.1% to 10% by decreasing holeescape time from 300 ps to 3 ps, and the hole mobility is decreased from180 cm²/V/s to 30 cm²/V/s. The foregoing modified parameters correspondto an SCH thickness of 100 nm. The carrier transport effect furtherdamps the ringing in the time series simulations of FIG. 6C.

For example, as illustrated in the time series simulations of FIG. 6Cand as compared to FIG. 6B, there is very efficient damping ofundershooting or ringing for the 1-to-0 transition as denoted at 602;indeed, ringing for the 1-to-0 transition is completely removed.However, there is relatively slow fall—as denoted at 604—which maycreate intersymbol interference (ISI). There is also slow rise for AM atthe 0-to-1 transition created by charging of the SCH, as denoted at 606.The damping for FM is not very efficient for the 0-to-1 overshootingbecause the speed of charging the SCH is slower than the relaxationoscillation frequency. As illustrated in the distribution simulations ofFIG. 6C and as compared to FIG. 6B, and as denoted at 608, the overflowin the p-side SCH increases even further for both electrons and holesdue to the relatively faster hole escape time and slower hole mobility.

The parameters for the simulation 600D of FIG. 6D are provided in TableD below.

TABLE D Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 0.1% escape time 300 ps electronmobility  2500 cm{circumflex over ( )}2/V/s hole mobility   30cm{circumflex over ( )}2/V/s electron tunneling time 0.134 ps holetunneling time  4.0 ps

Compared to the simulation 600C of FIG. 6C, the hole escape time isslower in the simulation 600D of FIG. 6D. In particular, the hole escapetime is increased from 3 ps to 300 ps, causing a decrease in the holeoverflow from 10% to 0.1%.

The slower hole escape time creates a very strong hole non-uniformity asillustrated by the Hole well distribution of FIG. 6D. Moreover, aninitial dip for both AM and FM in the time series simulations after aninitial rise at the 0-to-1 transition is more pronounced as compared toFIG. 6C.

The parameters for the simulation 600E of FIG. 6E are provided in TableE below.

TABLE E Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 10% escape time 3 ps electron mobility 2500 cm{circumflex over ( )}2/V/s hole mobility   30 cm{circumflex over( )}2/V/s electron tunneling time 0.134 ps hole tunneling time  4.0 ps

Compared to the simulation 600D of FIG. 6D, the carrier transport effectis weaker. For instance, the hole overflow is increased from 0.1% to 10%by decreasing hole escape time from 300 ps to 3 ps. The foregoingparameters in Table E correspond to an SCH thickness of 48 nm. As seenin the time series simulations, ringing is present because the SCH istoo thin, resulting in too much transient chirp, notwithstandingsubstantially uniform injection. In this example, the SCH is not thickenough to store sufficient charge to damp the ringing.

The parameters for the simulation 600F of FIG. 6F are provided in TableF below.

TABLE F Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 10% escape time 3 ps electron mobility 2500 cm{circumflex over ( )}2/V/s hole mobility   30 cm{circumflex over( )}2/V/s electron tunneling time 0.134 ps hole tunneling time  4.0 ps

The parameters in Table F for the simulation 600F of FIG. 6F areidentical to the parameters in Table E for the simulation 600E of FIG.6E. However, the simulation 600F of FIG. 6F considers the dynamic photonlifetime effect in the form of dynamic loss for the QWs. In thissimulation, optical loss due to IVBA may change dynamically. On thefall, e.g., on the 1-to-0 transition, N and S are in phase according tothe rate equations model 200 of FIG. 2A. A reduction of N reduces theIVBA loss, which is an effective increase in gain for lowering S. Thishas the same effect as gain compression, so damping increases comparedto FIG. 6E.

The parameters for the simulation 600G of FIG. 6G are provided in TableF below.

TABLE G Parameter: Value: electron capture time  1.0 ps hole capturetime  0.3 ps hole overflow → hole 0.1% escape time 300 ps electronmobility  2500 cm{circumflex over ( )}2/V/s hole mobility   30cm{circumflex over ( )}2/V/s electron tunneling time 0.134 ps holetunneling time  4.0 ps

Compared to the simulation 600F of FIG. 6F, the hole escape time isslower in the simulation 600G of FIG. 6G. In particular, the hole escapetime is increased from 3 ps to 300 ps, causing a decrease in the holeoverflow from 10% to 0.1%. Thus, the simulation of FIG. 6G combinesrelatively slow hole escape time with dynamic IVBA. The slower holeescape time creates a strong hole non-uniformity as illustrated by theHole well distribution of FIG. 6G. On the rise, e.g., on the 0-to-1transition, N and S are out of phase according to the rate equationsmodel 200 of FIG. 2A. As can be seen from the distribution simulations,when the stored charge in the SCH is high, the escape time makes adifference.

Accordingly, high-speed directly modulated DBR lasers may be configuredwith relatively thick SCH to damp ringing as described herein andthereby reach transmission distances of 20 km, 21 km, 25 km, or evenfurther. In contrast, conventional high-speed DMLs such as distributedfeedback (DFB) lasers typically used in DML applications may be limitedto a reach of 5-10 km.

FIG. 7 illustrates eye diagrams 702, 704 for a directly modulated DFBlaser and eye diagrams 706, 708 for a directly modulated DBR laser, suchas the DBR lasers described above. The thickness of SCH used for the DFBlaser was 60 nm. Both of the DFB laser and the DBR laser were modulatedat a data rate of about 10 G. The eye diagram 702 is a measured BB eyediagram and the eye diagram 704 is a measured 10.5 km eye diagram, bothfor the DFB laser. As illustrated in FIG. 7, the eye diagram 704 issubstantially closed after transmission through 10.5 km of dispersivefiber, leading to an unacceptably high bit error rate (BER).

The eye diagram 706 is a measured BB eye diagram and the eye diagram 708is a measured 21 km eye diagram, both for the DBR laser. As illustratedin FIG. 7, the eye diagram 708 is substantially open even aftertransmission through 21 km of dispersive fiber. The BER for the eyediagram 708 is about −31 dBm received power at a BER of 1×10⁻³ (FECrate). An avalanche photodiode (APD) was used as the receiver.

FIG. 8 illustrates receive power for various ERs (horizontal axis)changed by adjusting modulation amplitude for fixed DC bias for adirectly modulated DBR laser, such as the DBR lasers described above.The BER is fixed to 1×10⁻³ (FEC rate). An APD was used as the receiver.This is a common setup and condition for passive optical network (PON)applications. FIG. 8 also shows that a received power of −31 dBm is goodfor a BER of 1×10⁻³ after 25 km of transmission. The corresponding ER is7 dB.

With combined reference to FIGS. 7 and 8, the reason for the differencebetween the eye diagrams 704 and 708 after transmission for the DBRlaser vs. the DFB laser and the improved BER for the DBR laser is thatthe DFB laser exhibits relatively small damping and strong transientchirp while the DBR laser exhibits relatively high damping and weaktransient chirp, as illustrated in FIG. 9.

Damping factor F depends on the K factor of the laser according toΓ=Kf_(r) ²+cc, where f_(r) (also referred to herein as Fr) is relaxationoscillation frequency. The K factor may be calculated according to

$K = {\frac{4\pi^{2}}{v_{g}}{( {\frac{1}{\alpha_{tot}} + \frac{ɛ}{dg}} ).}}$In DBR lasers, ε occurs only in gain sections of the DBR laser and thereis no gain in passive sections of the laser, so dg is relatively less ina DBR laser than in a DFB laser. Accordingly, the K factor is relativelygreater for a DBR laser than for H a DFB laser. Further, Γ increaseswith increasing K factor. In some embodiments, both the K factor andf_(r) may be maximized for the DBR laser. f_(r) is proportional to thesquare root of the differential gain. After including both effects, onemay find an advantage of higher differential gain, small loss of cavity,and large gain compression factor to achieve large damping factor. Ahigh differential gain may be achieved by high quality growth of MQWstructure with strain, a use of InGaAlAs barriers to increase electronconfinement, for example. Lower loss of cavity can be achieved byincreasing a mirror reflectivity of the DBR laser and optimizingp-doping in an InP clad (also referred to herein as a cladding layer).To increase gain compression factor without degrading speed performance,the carrier transport effect can be used in a relatively thick SCH asalready described above.

As mentioned previously, the detune-loading effect may be leveraged toimprove the performance of a DBR laser, such as the DBR lasers alreadydescribed herein. In these and other embodiments, an effective alphaα_(eff) for the DBR laser may be reduced as the mode is tuned to thelonger wavelength side of a Bragg peak associated with the DBR laser.FIG. 10 is a graph depicting delay time, mirror slope, and detuneloading alpha, all as a function of wavelength, for an exampleembodiment of a DBR laser.

FIG. 11 is a graph depicting measured FM efficiency as a function ofphase condition of an example embodiment of a DBR laser. Phase currentsin a range from about 1-2 mA and in a range from about 9-11 mA showhysteresis regions where short-wavelength mode (high FM) is selected asphase current is ramped up from low to high current whilelong-wavelength mode (low FM) is selected as phase current is rampeddown from high to low phase current. Stable single mode operation wasobtained from a phase current range of about 2-8 mA. For phase currentin a range from about 2-4 mA, lasing happened on the longer wavelengthside of the corresponding Bragg peak, and the corresponding FMefficiency is lower. Although not shown in this graph, the correspondingFr is higher, as is expected from the detune-loading effect, in thisoperating condition. Such operation condition may be applied forextending the reach by direct modulation of the DBR laser because 1. FMefficiency is lower, 2. Fr is higher, and therefore transient chirp canbe suppressed to a greater extent.

In view of the foregoing, some embodiments described herein include adirectly modulated DFB and/or DBR laser configured to operate at a datarate of about 10 G or higher with a reach of 25 km and which may be usedin high-power PONs. To obtain a reach of 25 km with an acceptable BER at10 G in a directly modulated DFB and/or DBR laser, transient chirp maybe reduced compared to the transient chirp in a conventional 10 G DFBlaser as already described herein.

Alternately or additionally, the DFB and/or DBR laser may include one ormore of the following features:

First, the DFB and/or DBR laser may have a relatively high bias with abias current density of about 0.2 mA/um or higher, e.g., 60 mA bias for300 um gain length (1.2 um mesa width) or higher. Biasing the laser witha high bias may increase the speed of the laser and reduce the dampingof the laser. In these and other embodiments, the direct modulationsource—such as a modulation driver—driving the laser may include alow-power-consumption linear amplifier which can generate a relativelylarge modulation swing of 60 mApp-90 mApp so that the laser can bebiased high while still having a good ER.

Second, the laser may be a fast laser. For instance, some embodimentsinclude a 10 G laser with a bandwidth (BW) of 20 GHz.

Third, the laser may include a thick SCH to induce carrier transporteffect. For example, the SCH may have a thickness of between about 60nanometers (nm) to about 125 nm, or about 100 nm in some embodiments. Incontrast, other DFB and DBR lasers may have an SCH with a thickness ofless than about 50 nm. The thicker SCH provided in the laser accordingto some embodiments may increase damping of relaxation oscillation andreduce chirp as compared to lasers with relatively thinner SCH.

Fourth, the laser may be a DBR laser as opposed to a conventional DFBlaser with relatively thin SCH typically used in DML applications. TheDBR laser may have relatively more passive section(s) than the DFBlaser, which passive section may increase the damping of the DBR laseras compared to the damping of the DFB laser as described above.

Fifth, the detune-loading effect can be applied such that the phasecondition (e.g., the lasing wavelength) of the DBR laser is tuned towardthe long wavelength side of the Bragg peak associated with the DBR laserso as to increase the speed of the DBR laser. While doing so may reducethe damping factor, the benefit from the improvement in speed mayoutweigh the penalty from the reduction in the damping factor.

Sixth, a DBR can be provided on each side of the gain section of the DBRlaser (e.g., in the front-rear configuration of FIG. 1C) to improve theyield of single mode operation. Monitoring photo currents from the DBRscan be used to monitor output power. By measuring the ratio of photocurrents from the front and rear DBRs, it may be possible to find thephase condition of lasing and avoid mode hop. Accordingly, embodimentsdescribed herein may involve adjusting the SCH thickness and/or carriertransport which changes internal, e.g., photon lifetime. By acombination of the foregoing, ringing and/or peaking may be adjusted toprovide an appropriate amount of damping to extend the reach of directlymodulated DBR laser.

When ringing is damped, the DBR laser may become a substantially lineardevice, which may be beneficial in analog applications where suppressingdistortions is desirable. Applications in which a substantially lineardirectly modulated DBR laser may be implemented include discretemultitone (DMT) modulation and analog applications, among potentiallyothers.

As described herein, the use of DBR lasers in DML applications mayenable a reach of about 20 km or longer at 10 G data rates. The ER oflasers typically used in DML applications may be limited to about 5 dB,although some embodiments of the directly modulated DBR lasers describedabove may have an ER of between about 6.5-7 dB due to high modulationamplitude of current. Moreover, the power of directly modulated DFBlasers and/or DBR lasers may be somewhat limited in some embodiments dueto a finite front facet reflectivity to form a laser cavity and by theavailable amplitude of modulation swing. Accordingly, some embodimentsdescribed herein may integrate an optical amplifier in a laser chip withsemiconductor laser such as a DFB laser or a DBR laser to boost powerand/or ER of the optical signals emitted from the chip as compared tooptical signals emitted by laser chips lacking an integrated opticalamplifier.

Accordingly, FIG. 12A illustrates an example embodiment of a laser chip1200 including a DBR laser 1202 and an integrated optical amplifier1204, arranged in accordance with at least some embodiments describedherein. According to some embodiments, the DBR laser 1202 and theoptical amplifier 1204 may be co-modulated with the same modulationsignal. Although the laser chip 1200 is illustrated as including a DBRlaser integrated with an optical amplifier, in other embodiments thelaser chip includes a DFB laser integrated with an optical amplifier.

In general, the DBR laser 1202 may be configured similar to the DBRlasers described above. For instance, in the illustrated embodiment, theDBR laser 1202 includes a gain section 1206 and a passive section 1208,similar to the DBR lasers 100, 110, 120 of FIGS. 1A-1C. The gain section1206 may include an active region such as an MQW region which may bepositioned between an upper and lower SCH and may otherwise beconfigured according to the embodiments described above. The passivesection 1208 may include a DBR.

The optical amplifier 1204 may include, but is not limited to, asemiconductor optical amplifier (SOA), a multimode interference (MMI)SOA, or a Mach-Zehnder (MZ) modulator. For example, FIG. 12B illustratesan example embodiment of the laser chip 1200 of FIG. 12A in which theoptical amplifier 1204 includes a SOA including an MQW region 1210. Inthis and other embodiments, the passive section 1208 is positionedbetween the gain section 1206 and the optical amplifier 1204 and acoupling between the passive section 1208 and the optical amplifier 1204may include a butt joint coupling having an angle off normal incidenceof an incoming optical signal generated by the DBR laser 1202.

FIG. 12B further illustrates a MQW region 1212 and a DBR 1214 that maybe included in, respectively, the gain section 1206 and the passivesection 1208 of the DBR laser 1202. In these and other embodiments, thelaser chip 1200 may further include a common guiding structure withinthe DBR laser 1202 and the optical amplifier 1204, such as a shallowridge or a buried heterostructure. In an example embodiment, the gainsection 1206 may have a length of about 300 um and the optical amplifier1204 may have a length of about 200 um.

In addition, the laser chip 1200 includes an HR coating 1216, a firstelectrode 1218 coupled to the gain section 1206 of the DBR laser 1202, asecond electrode 1220 coupled to the optical amplifier 1204, and an ARcoating 1222. In this and other embodiments, the first and secondelectrodes 1218, 1220 may be coupled to a common direct modulationsource 1224. Accordingly, the gain section 1206 and the opticalamplifier 1204 may be modulated with a common modulation signal suppliedby the common direct modulation source 1224.

FIG. 12C illustrates an example embodiment of the laser chip 1200 ofFIG. 12A in which the optical amplifier 1204 includes an MMI SOA 1226.FIG. 12C additionally illustrates a first electrode 1228 coupled to thegain section 1206 of the DBR laser 1202 and a second electrode 1230coupled to the optical amplifier 1204. Although not illustrated in FIG.12C, the first and second electrodes 1228, 1230 may be coupled to acommon direct modulation source, such as the common direct modulationsource 1224 of FIG. 12B.

FIGS. 12D and 12E illustrate example embodiments of N-arm MZ modulators1232, 1234, arranged in accordance with at least some embodimentsdescribed herein. In particular, the N-arm MZ modulator 1232 is a 2-armMZ modulator, while the N-arm MZ modulator 1234 is a 3-arm MZ modulator.More generally, the number N of arms may be greater than 1. The N-arm MZmodulator 1232 of FIG. 12D may be included in the optical amplifier 1204of FIG. 12C, e.g., in place of the MMI SOA 1226. Analogously, the N-armMZ modulator 1234 of FIG. 12E may be included in the optical amplifier1204 of FIG. 12C, e.g., in place of the MMI SOA 1226.

SOAs under DC may distort AM optical signals. In particular, high biasto the SOA can cause AM peaking in the leading edge of relatively higherintensity 1 bits compared to lower intensity 0 bits, as denoted in FIG.13 at 1302 in an AM profile 1304. The AM profile 1304 of FIG. 13 wasobtained at an SOA bias of 60 mA DC, for an AM optical signal generatedby a DBR laser biased at 58 milliamperes (mA) with a modulation swing of90 mApp and at a data rate of about 10 G. FIG. 14 illustrates an AMprofile 1402 and an FM profile 1404 of an optical signal generated by adirectly modulated laser after passing through a SOA under the sameconditions as described with respect to FIG. 13. Under the foregoingconditions, the SOA induces linear red chirp across each pulse in theoptical signal as shown by the FM profile 1404 of FIG. 14, which chirprecovers at a time constant corresponding to carrier lifetime.

FIG. 15 illustrates 3 cases for operation of a DBR laser and a SOA,arranged in accordance with at least some embodiments described herein.In case 1, an AM optical signal generated by a DML, such as a directlymodulated DBR laser, passes through a DC-biased SOA. If the SOA is inthe saturation regime (e.g., close to maximum power), the SOA causesdistortion by amplifying only the leading edge efficiently.

In case 2, a continuous waveform (CW) optical signal passes through theSOA and the SOA is modulated. Usually, the speed of the SOA is not fastenough for a data rate of 10 G or higher due to limited speed of carrierlifetime. As a result, the intensity of the falling edge increases sincethe speed of the SOA is relatively slow.

In case 3, the DBR laser and the SOA are co-modulated with the same datasignal such that the peaking at the leading edges as in case 1 may beneutralized by the peaking in the falling edges as in case 2 to generatean ideal or at least improved response, as generally illustrated in Case3 and corresponding to the configuration of the laser chip 1200 depictedin FIG. 12B above. The peaking at the leading edges may be neutralizedby the peaking in the falling edges since the time constant of case 1and the time constant of case 2 naturally match as both are determinedby carrier lifetime. In some embodiments, the high power injection oflight into the SOA may increase the modulation speed of the SOA.Additionally, the ER of the output signal may be higher than the ER ofthe optical signal prior to transmission through the SOA if the depth ofthe co-modulation is optimized; otherwise, the ER of the output signalmay be degraded.

A laser chip including a DBR laser and integrated optical amplifier,such as the laser chip 1200 described herein, can be tested under cwconditions. However, such testing may not indicate how the resultingwaveform may look dynamically. Accordingly, FIG. 16 is a graphindicating operating conditions that can be used to avoid ER degradationafter the SOA. For example, for cw bias to the SOA (e.g., no modulationto SOA is used), one fixed bias case for the SOA (having a length of 200um in this case) can be analyzed. At 2 mA, the SOA may becometransparent. Below 2 mA (e.g., 0 V or 0.5 V, below PN junction turn-onvoltage, Vth=0.7 V), the SOA can function as a saturable absorber (e.g.,saturation of absorption). When gain bias is increased, at the beginning(e.g., <50 mA gain, for 0V bias to SOA, see blue curve) there is nooutput because the SOA absorbs light. Absorption will be bleached whenincoming power into the SOA exceeds certain power. The output powerlinearly increases for gain bias >60 mA in this case.

When the SOA bias is higher than 10 mA, significant saturation of gainis observed. Output power increases steeply at the beginning for lowbias of gain, however, it quickly saturates (roll-over) for high gainbias. If the gain section of the DBR laser in a laser chip such as thelaser chip 1200 is modulated, and the SOA is biased under cw condition,the ER at the output of SOA will degrade due to gain saturation in theSOA. To avoid ER degradation in the SOA in the gain saturation regime,according to some embodiments described herein, the SOA bias ismodulated to higher current for the higher input (1 bits) while it isreduced for the lower input (0 bits). This is a so called “push-push”modulation scheme. In terms of module design, this can be simplyimplemented by splitting RF current into the gain section and the SOA(or other optical amplifier), and therefore, is a convenient modulationscheme to design a compact transmitter optical subassembly (TOSA)module.

For a PON application which requires 5-9 dBm fiber coupled power (calledPR40 for EPON), the power of a laser chip including a DBR laser with a300 um gain section length biased at 60 mA+SOA of length 200 um biasedat 20 mA is high enough, as in this example. Use of a shorter devicedesign, for example, 150 um gain section length for the DBR laser and200 um SOA length with 20 mA bias, may enable a device packaged into atransistor outline (TO) CAN having limited heat load handling capabilityof a thermoelectric cooler (TEC). In particular, a laser chip such asthe laser chip 1200 may be packaged with or without a TEC in a TO CAN.

The SOA length can be modified to be short enough (e.g., <300 um) toavoid undesirable distortion toward the end of the SOA. The shorterlength of the SOA may be advantageous for suppressing distortion duringamplification, although the gain of the SOA may be reduced.

High incident power into the SOA can improve the speed of the SOA byreducing the carrier lifetime of the SOA via faster stimulated emissiontime.

The facet reflectivity of the SOA can be finite or nearly 0%. Very lowfacet reflectivity on the SOA may be realized by use of an angledwaveguide or window structure and may avoid complex modulation in thepassive section caused by delayed feedback effect (e.g., self-seedingfrom close reflection plane), which is sometimes not desirable. When thephase condition of feedback is precisely controlled, however, thefeedback effect can be used to damp relaxation oscillation to extend thetransmission distance. One drawback is that such a condition can besensitive to the phase condition of reflected light and may needfeedback loop control. To avoid this drawback, two methods can be used.One is simply to eliminate the reflection at the facet as alreadymentioned. The other design is to use an angled waveguide at the outputof the SOA to eliminate reflection from the facet, but add very lowreflectivity grating near the output. In this case, the phase offeedback can be controlled by the grating phase and is morereproducible.

Thus, in a laser chip including a DBR laser and an integrated SOA suchas is illustrated in FIG. 12B above, a higher ER after the SOA comparedto the ER before the SOA can be achieved by simultaneously modulatingthe SOA with the same data modulating the gain section of the DBR laser.Simultaneously, the optical power after the SOA compared to the opticalpower before the SOA can be increased by the gain of the SOA.

Alternately, in the embodiment of FIG. 12C in which the laser chip 1200includes a DBR laser 1202 and an MMI SOA 1226, the use of the MMI SOA1226 increases the saturation output power compared to a non-MMI SOAsince the photon density in the MMI gain section (e.g., the SOA) isreduced compared to the photon density at the input due to spreading ofthe mode area. The same principle applies to a 1×N or N-arm MZ modulator(N>2) such as is illustrated in FIGS. 12D and 12E. The MMI SOA and/orthe arms of the MZ modulator can be modulated in order to enhance the ERof the output signal compared to the input signal through interferenceof modes in the case of the MMI SOA or interference of recombined signalportions received from the various arms in the case of the 1×N MZmodulator.

The embodiments of FIGS. 12B-12E may include one or more of thefollowing aspects, as applicable:

First, the guiding structure for both the DBR laser 1202 and the SOAincluded in the optical amplifier 1204 can include a shallow ridge or aburied heterostructure which is the same for both the DBR laser 1202 andthe SOA to reduce coupling loss.

Second, the SOA can be used for the MMI gain section where the mode areais spread out, and therefore photon density is relatively lower, toavoid gain saturation of the SOA.

Third, a coupling between a passive section of the chip and the SOA gainmaterial can include a butt joint coupling. The butt-joint coupling canhave an angle off normal incidence of an incoming beam to avoidreflection.

Fourth, the electrode for an MMI SOA can be confined to the area wherethe mode is spread out.

Fifth, the MMI SOA can be co-modulated with the same data as the DBRlaser similar to the description already provided with respect toconfigurations such as illustrated in FIG. 12B.

Sixth, in order to further enhance the ER at the output of an MZmodulator, one or more of the following may be implemented in thecorresponding laser chip including a DBR laser and integrated opticalamplifier:asymmetric splitting ratio in a corresponding MMI device, asection to control the loss in each arm (e.g., of an MZ modulator), orasymmetric DC bias or RF modulation to the SOA.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations may be expressly set forth herein for sakeof clarity.

It will be understood by those within the art that, in general, termsused herein, and especially in the appended claims (e.g., bodies of theappended claims) are generally intended as “open” terms (e.g., the term“including” should be interpreted as “including but not limited to,” theterm “having” should be interpreted as “having at least,” the term“includes” should be interpreted as “includes but is not limited to,”etc.). It will be further understood by those within the art that if aspecific number of an introduced claim recitation is intended, such anintent will be explicitly recited in the claim, and in the absence ofsuch recitation no such intent is present. For example, as an aid tounderstanding, the following appended claims may contain usage of theintroductory phrases “at least one” and “one or more” to introduce claimrecitations. However, the use of such phrases should not be construed toimply that the introduction of a claim recitation by the indefinitearticles “a” or “an” limits any particular claim containing suchintroduced claim recitation to embodiments containing only one suchrecitation, even when the same claim includes the introductory phrases“one or more” or “at least one” and indefinite articles such as “a” or“an” (e.g., “a” and/or “an” should be interpreted to mean “at least one”or “one or more”); the same holds true for the use of definite articlesused to introduce claim recitations. In addition, even if a specificnumber of an introduced claim recitation is explicitly recited, thoseskilled in the art will recognize that such recitation should beinterpreted to mean at least the recited number (e.g., the barerecitation of “two recitations,” without other modifiers, means at leasttwo recitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,etc.” is used, in general such a construction is intended in the senseone having skill in the art would understand the convention (e.g., “asystem having at least one of A, B, and C” would include, but not belimited to, systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, etc.). In those instances where a convention analogous to “atleast one of A, B, or C, etc.” is used, in general such a constructionis intended in the sense one having skill in the art would understandthe convention (e.g., “a system having at least one of A, B, or C” wouldinclude, but not be limited to, systems that have A alone, B alone, Calone, A and B together, A and C together, B and C together, and/or A,B, and C together, etc.). It will be further understood by those withinthe art that virtually any disjunctive word and/or phrase presenting twoor more alternative terms, whether in the description, claims, ordrawings, should be understood to contemplate the possibilities ofincluding one of the terms, either of the terms, or both terms. Forexample, the phrase “A or B” will be understood to include thepossibilities of “A” or, “B” or “A and B.”

In addition, where features or aspects of the disclosure are describedin terms of Markush groups, those skilled in the art will recognize thatthe disclosure is also thereby described in terms of any individualmember or subgroup of members of the Markush group.

As will be understood by one skilled in the art, for any and allpurposes, such as in terms of providing a written description, allranges disclosed herein also encompass any and all possible subrangesand combinations of subranges thereof. Any listed range can be easilyrecognized as sufficiently describing and enabling the same range beingbroken down into at least equal halves, thirds, quarters, fifths,tenths, etc. As a non-limiting example, each range discussed herein canbe readily broken down into a lower third, middle third and upper third,etc. As will also be understood by one skilled in the art all languagesuch as “up to,” “at least,” and the like include the number recited andrefer to ranges which can be subsequently broken down into subranges asdiscussed above. Finally, as will be understood by one skilled in theart, a range includes each individual member. Thus, for example, a grouphaving 1-3 cells refers to groups having 1, 2, or 3 cells. Similarly, agroup having 1-5 cells refers to groups having 1, 2, 3, 4, or 5 cells,and so forth.

From the foregoing, it will be appreciated that various embodiments ofthe present disclosure have been described herein for purposes ofillustration, and that various modifications may be made withoutdeparting from the scope and spirit of the present disclosure.Accordingly, the various embodiments disclosed herein are not intendedto be limiting, with the true scope and spirit being indicated by thefollowing claims.

The present invention may be embodied in other specific forms withoutdeparting from its spirit or essential characteristics. The describedembodiments are to be considered in all respects only as illustrativeand not restrictive. The scope of the invention is, therefore, indicatedby the appended claims rather than by the foregoing description. Allchanges which come within the meaning and range of equivalency of theclaims are to be embraced within their scope.

What is claimed is:
 1. A laser chip comprising: a laser including anactive region; an optical amplifier integrated in the laser chip infront of and in optical communication with the laser; a first electrodeelectrically coupled to the active region; and a second electrodeelectrically coupled to the optical amplifier; wherein the firstelectrode and the second electrode are configured to be electricallycoupled to a common direct modulation source, wherein the common directmodulation source is configured to supply a modulation signal having adata rate of about 10 gigabits per second or higher, wherein in responseto application of the modulation signal to the first electrode, the DBRlaser is configured to generate an optical signal having a frequencymodulation profile exhibiting both transient chirp and adiabatic chirp,a ratio of transient chirp to adiabatic chirp being in a range from 1:3to 1:4.
 2. The laser chip of claim 1, wherein the optical amplifiercomprises a semiconductor optical amplifier (SOA).
 3. The laser chip ofclaim 2, further comprising a common guiding structure within the laserand the SOA.
 4. The laser chip of claim 3, wherein the common guidingstructure comprises a shallow ridge or a buried heterostructure.
 5. Thelaser chip of claim 1, wherein the optical amplifier comprises amultimode interference (MMI) semiconductor optical amplifier (SOA). 6.The laser chip of claim 1, wherein the optical amplifier comprises anN-arm Mach-Zehnder (MZ) modulator where N>1.
 7. The laser chip of claim1, wherein the active region and the optical amplifier are modulatedwith a common modulation signal supplied by the common direct modulationsource.
 8. The laser chip of claim 7, wherein peaking in a leading edgeof an amplification response of the optical amplifier in an absence ofapplication of the common modulation signal to the optical amplifier isat least partially neutralized by peaking in a falling edge of theamplification response of the optical amplifier in response toapplication of the common modulation signal to the optical amplifier. 9.The laser chip of claim 1, wherein an extinction ratio (ER) of a firstamplitude modulation (AM) optical signal generated by the DBR laser andreceived by the optical amplifier is less than an ER of a second AMoptical signal output by the optical amplifier.
 10. The laser chip ofclaim 1, wherein the laser chip is packaged in a transistor outline (TO)CAN.
 11. The laser chip of claim 10, further comprising a thermoelectriccooler (TEC) packaged with the laser chip within the TO CAN.
 12. Thelaser chip of claim 1, wherein the laser comprises a distributed Braggreflector (DBR) laser including: a gain section that includes the activeregion; and a passive section comprising a Bragg filter in opticalcommunication with the active region.
 13. The laser chip of claim 12,wherein: the optical amplifier comprises a semiconductor opticalamplifier (SOA); the passive section of the DBR laser is positionedbetween the gain section of the DBR laser and the SOA; and a couplingbetween the passive section of the DBR laser and the SOA comprises abutt joint coupling having an angle off normal incidence of an incomingoptical signal generated by the DBR laser.
 14. The laser chip of claim12, wherein the gain section has a length of about 300 micrometers (um)and the optical amplifier has a length of about 200 um.
 15. The laserchip of claim 12, wherein the gain section of the DBR laser furtherincludes: an upper separate confinement heterostructure (SCH) above theactive region having a thickness of at least 60 nanometers (nm); and alower SCH below the active region having a thickness of at least 60 nm.16. The laser chip of claim 15, wherein the thickness of the upper SCHis less than 125 nm and the thickness of the lower SCH is less than 125nm.
 17. The laser chip of claim 12, wherein the DBR laser is tunedtoward a long wavelength side of a Bragg peak associated with the DBRlaser.